Additive or subtractive synthesis ?
Before trying to measure the number of colors that a human being is able to see, let's try to understand how it perceives colors since it does not have a cone that is sensitive to each color. To do this, let's analyze how a screen and a printer work.
A monitor or printer does not work with colors in the same way : the screen works with so-called additive colors - Red, Green, Blue - or RGB to give white by addition as shown in the center of this color animation below - while a printer works in subtractive mode - Cyan, Magenta, Yellow, Black or CMYK to give black by subtraction - (below). Well, it turns out that the human eye works a little like a screen (it's an image of course!), i.e. in an additive mode.
In concrete terms, with a display that works in additive mode, if no RGB signal is sent to the pixels (0, 0, 0), the display remains black. It is black by default. To obtain white (255, 255, 255) it is necessary to illuminate each Red, Green and Blue pixel, as shown in the figure opposite at most. The other colours are obtained by changing the value of each primary colour. To display yellow for example (255, 255, 0), you must stop illuminating the blue pixels (the screen is yellow at the intersection between green and red because the blue pixels are off). Only three colors require only one pixel. The three primary colours and their variations in brightness (from 0 to 255). All the others, i.e. the millions of others, will be a mixture of at least two pixels of more or less luminous primary colors.
With a printer, you start from an already white sheet of paper by default because it reflects all wavelengths of visible light in equal parts. So there, it is the opposite, if we want black we will have to project ink with complementary colours on this sheet, i.e. Cyan, Magenta and Yellow which have the particularity of absorbing each time a part of the visible light. In practice it will also be necessary to add Black ink because 100% of the other three colours do not produce a deep black because of the impurities contained in the inks but a very dark brown. It is very clear from the image opposite that the centre is brown and not black. These inks projected in the same place will have the property of absorbing all the luminous radiations. So this place will look black now. If you want the paper to look blue, you have to project magenta and cyan on it in the same place. All light radiation, except blue, will be absorbed. Only the blue wavelengths of the light will be reflected at this location.
By convention, it has been decided by the International Commission on Illumination - CIE - that the primary red colour is light with a wavelength of 700 nm, green 546 nm and blue 436 nm. Whatever the wavelength of their RGB pixels, screen manufacturers still try to reproduce colours as well as possible...
How many colors are perceived by the human eye ?
It remained to be seen how many colored shades the human eye could perceive in these famous three so-called primary colors. This work was carried out in the 1930s under the aegis of the Compagnie Internationale de l'éclairage (La CIE). She asked very many people to carefully compare shades of blue, green and red to count how many different colours, on average, they were able to distinguish. Two important things emerged from these test series.
- Human beings with the finest visual acuity are able to distinguish up to 200 shades per color,... but
- never in the three primary colors !
- And the gamut of the "average" eye is very wide : the famous L*a*b* space.
So if a simple arithmetic calculation gives us 200 x 200 x 200 or 8 million different perceived colors, in reality we are very far from that because color specialists (researchers, color marketing professionals) tell us that 300,000 (three hundred thousand) would be much more realistic ! If a high-performance eye is indeed able to distinguish up to 200 shades of the same primary colour, the same eye never achieves this feat in the other two primary colours.
Conclusion on color perception : Certainly, the human eye is sometimes able to distinguish many shades for a given color, but overall, a significant difference between two shades of the same color is required so that, without ambiguity, this person can affirm that he or she has indeed perceived two different colors. This does not prevent him from perceiving a very important and wide gamut, the famous L*a*b* space. We are beginning to guess that several RGB combinations will code for the same color perceived by an individual...
It is now time to know how a colour is described "computerically". Typically, colours are described using three or four digits depending on whether you work in RGB or CMYK, always with the objective of reproducing the functioning of the human eye. Thus such color will be noted - 158, 128, 84 - in RGB and - 80%, 64%, 21%, 50% - in CMYK. But why are these numbers and to which colour does it correspond exactly ?
The next page is about how computers treat these colors when they are only able to work with 0s and 1s - Colors and computer science - 3 / 10